Note the character of the difference of the two sendings of the blessed Spirit in John 14, and 15, 16. In the first, as often noted, the Father sends in His Name. It is their comfort and sustaining; One who comes instead of the One they were losing; leads them into all truth; all the truth, excellent and heavenly things; would bring Christ to them in Spirit, so that they would not be comfortless. He would bring all things to their remembrance which they had heard of Him while here, and teach them all things. He would stay with them, which Christ could not; and be in them, which Christ as then present could not.
When Christ sends Him from the Father it is more in testimony: "He shall testify of me," the glorified Man; would guide them into all the truth; show them things to come; bring down heavenly things, what He heard, when come down here; and show them things to come down here: the double testimony, take the things of a glorified Christ, and show them to them; and all that the Father had was His. Heavenly truth, prophetic truth, and the glory possessed by Christ, that is, all that the Father had; these are the three subjects of testimony; chapter 15:26, 27, gives the double subject of testimony generally, testifying of Christ exalted in glory, and bringing to remembrance all that He had said to them on earth.
Note, too, how markedly He is spoken of as down here. Whatsoever He hears, that shall He speak. He is in the place of a servant in the apostles, as Christ was; only it was not incarnation, of course; but He was hearing, and speaking down here. But chapter 15: 26, 27, gives His business down here, verse 27 depending on chapter 14: 26. But chapter 15: 26, 27, makes it much more personal and blessed, and engaged their personal affections in their testimony. This would not apply to Paul, of course. He only speaks of His humiliation as part of the immense scheme of truth and of grace.